Lallana was given his marching orders after reacting to a series of strong challenges by grabbing Tottenham midfielder George Marsh by the neck.
After being dragged away by Danny Ings, the 29-year-old left the field and left the U23s a man down as they fell to a 1-0 defeat in Stevenage.
This proved a major dampener on a night that saw Lallana start in a bid to regain match fitness and reclaim his first-team spot.
“We wanted him—Alberto Moreno as well but he had a little problem [that kept him out]—to have match time,” he told reporters at Melwood.
“He had 60 minutes. It’s not nice, I spoke to him, it’s now how it should be.
“He’s not happy about it, but I understand why it happened. That’s what I thought in the first moment: it wasn’t the first challenge [on Lallana].
“Adam is not happy about the reaction, as I was not happy about it, but it doesn’t change my mind about Adam.
“He is on the way back, that’s how it is again. We will not rush it, for sure.
“He’s for sure an option for the squad [at Southampton], if he’s an option for the first 11 I would rather think not.
“But I can sleep twice before we play, so maybe I will change my mind. He’s in really good shape, fresh, looks quick, but [setbacks] have happened twice.
“Hopefully [there are] around about 18, 19, 20 games to go for us, and that’s enough for him as well.”